Evolutionary insights caught on camera

Spying on wild crickets in the field yields secrets of reproductive success.

Janelle Weaver

Tom Tregenza and his colleagues have closely followed field crickets in the wild.T. Veen

By monitoring the sex lives of wild crickets using surveillance cameras and working out their parentage using DNA analyses, scientists have tested some key assumptions of evolutionary theory in a natural setting.

Males of many different species fight one another for access to females and show off with ornamental displays, and females often choose mates with attractive traits that indicate their offspring will be viable and healthy. Crickets are no exception to the prevailing pattern, at least in the lab: males compete to monopolize females and impress them with their songs, and females in turn favour dominant males.

Although scientists regularly track the mating patterns of animals in the wild, most insect observations have taken place in the lab because they are more difficult to follow in their natural habitat. Because organisms in the lab don't experience the same selection pressures as those in the wild, it's important to test theories of evolution in natural settings.

Evolutionary ecologist Tom Tregenza of the University of Exeter in Penryn, Cornwall, set out to discover whether lab results for reproductive success in crickets would be the same in wild populations. He and his colleagues report their findings today in the journal Science1.

Cricket cams

Tregenza's team continuously monitored a population of more than 150 wild, flightless field crickets (Gryllus campestris) in a meadow in northern Spain for two years using a network of 64 motion-sensitive, infrared-equipped video cameras placed near burrow entrances. Crickets wore unique identification tags and mated in full view near burrow entrances. Because crickets live for about a year, the team was able to track parents and their offspring for their entire lifespan. To assign parentage, they trapped crickets, cut off the tip of their hind legs, extracted DNA and compared the genetic similarities of 11 molecular markers.

Male (left) and female crickets outside their burrow in a field in northern Spain.T. Tregenza & R. Rodríguez-Muñoz

The team found that females and males had similar numbers of mates, indicating that females aren't as choosy as previously thought. Female crickets only have to mate once because they can store sperm, but the team found that the more the females mated, the more offspring they had. This finding challenges the assumption that it is costly for females to mate too much, says evolutionary biologist William Wagner, who studies crickets at the University of Nebraska–Lincoln. "Conflicts over mating may not be as common as we think they are," he says.

Similar to the males, many of the females did not leave any descendants. "That is striking," says Darryl Gwynne, who studies insect evolution at the University of Toronto at Mississauga, Ontario, Canada. Eggs are a limiting resource in which females must invest energy to produce, he explains. "If that is replicated by other studies, it will force us to re-examine sexual selection theory and how it pans out in natural animal populations."

Evolution in action

The study revealed other complex and surprising results. Dominant males had fewer mates than subordinate males, but they had similar numbers of offspring. Smaller males relied on singing to attract mates and have offspring, whereas singing did not affect the number of mates for larger males. Similarly, short-lived males relied more on singing to sire offspring than long-lived males. These patterns are unexpected, Wagner says, because scientists had thought that singing plays a strong role in reproductive success for all males.

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The results suggest that to understand evolution, it is important to verify lab findings in natural settings and examine interactions among multiple traits, such as size, lifespan, dominance and sex. "It's not as simple as people would think," says Anna Qvarnström, an evolutionary biologist at Uppsala University in Sweden.

In future studies, Tregenza would like to determine whether the factors that affect success at mating and producing offspring stay the same or vary from year to year, depending on environmental conditions. For example, different genetic traits may confer advantages in cold temperatures compared to hot temperatures. Using this system for monitoring multiple generations of wild populations, Tregenza hopes to observe evolution in action.